The forum
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Refocusing the General Chemistry Curriculum
Why Should They Know That? Stephen J. Hawkes Oregon State University, Colvallis, OR 97331-4003 Curriculum changes usually start from a n assumption that somebody knows what should be taught, or that it is necessarily a matter of opinion and can only represent a consensus. It is my contention that consensus reflects only the training and preconceptions ofthe persons reaching it. The present investimtion seeks obiective criteria on which to construct a n intmductory chemistry curriculum, by considering what the curriculum prepares students to do. This paper addresses four propositions: 1. Nobody knows what aspects of introduda2y chemistry are actually valuable to students, or which of many "fundamental" concepts they will find most useful. 2. It is possible to fmd out what is valuable to them only by careful research and painful honesty. 3. Much of what we now teach is incorrect or more approxi-
mate than we lead students to believe. 4. Even more is of too little value to students to justify their
study time. Since many of my readers will believe that "it ain't broke" and should therefore not be fixed, I shall take these in the reverse order and show first that it's broke. Not Sufficiently Valuable The subiect I have most enioved teaching is the allotrow of sulfur. i t is fun to demonstrate the various transformations. toss ~ i e c e sof olastic sulfur to the students, and explain' the molecular basis of those changes. Yet when the student reviews this material for an exam he or she has a series of changes of no intrinsic importance, illustrating principles that would be illustrated just as well by more important and familiar substances, such as the alkanes from methane to polyethylene and the transition from graphite to diamond, or ice-I to ice-111. As an undergraduate I had reason tobe interested in the chromate-dichromate equilibrium. When I became a teacher I was disappointed to find that students did not share my interest. Why should they? A chemist regularly involved with chromium would be interested, but few people have any reason to be involved with chromates even if they are chemists, now that Cr(VI) is avoided as an oxidant. The hydrolysis or condensation of organic compounds is of more immediate interest to more people than the hydrolysis or condensation of chromates. This is especially true if the organic compounds are biomolecules such as proteins and carbohydrates. Almost all one-year introductory texts derive the Bragg equation. Yet the one biochemistry text that I have consulted gives a five-page introduction to X-ray diffraction without mentioning the Bragg equation and deals only briefly and qualitatively with diffraction by ionic crystals Based on an address at the 201st American Chemical Society Meeting in Atlanta, GA, April 1991, as part of the FlPSE Symposium
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Journal of Chemical Education
before passing on to fibers andmolecular crystals. Concentrating the students'attention on diffraction by such wellunderstood crvstals as NaCl deflects it from the ~roblems nowadays adiressed in crystallographic reseakh. The mathematics of such com~lexdiffraction patterns as those of DNA or the new cera2cs is beyond mist students, but the concept is not and the conclusions relate to phenomena that the students can relate to. Incorrect or Unreliable Solubility product calculations a t the level taught in introductory chemistry almost always give dramatically incorrect figures for solubility equilibria ( I ) , often failing even of a "ball park approximation". Introductory students could be taught to use computer programs to work out such equilibria if this could be shown to be a useful skill, hut a qualitative understanding is all that is useful to any but a tiny minority of our students (2).These few (e.g., graduate students in soil science) will be taught how to do it right in later courses. Conversely, solubility product is seldom presented as a balance between crystal and solution forces. Tables of K,, in our texts contain values for only one crystalline form, even when two or three exist. This wastes the opportunity to teach the meaning of K,,. The fundamental aspects of solution theory are thus neglected in favor of calculations that have little relation to reality. Mathematical models of the chemical bond are vital in the work of chemical physicists, but have little or no usefulness in introductory chemistry. Resonance was an elegant tool for use with the mechanical calculators of the 1930'9, but it is now obsolete in chemical physics. Its qualitative descriptionin the early chapters of our texts cannot be used to predict anything useful to our students. I have regularly made them use it to decide whether bond length is longer in NOz- or NOS-, while wondering why they should care and why I didn't use a simpler way of deciding. It is often said that a molecule is "stabilized by resonance" but this is mysticism. Resonance is a mental concept and cannot stabilize anythmg. Benzene is stable because the six electrons not involved in the single C-C or C-H bonds combine into a single six-electron cloud extending around the ring. The representation of a double bond as a combination of a sigma and a pi bond and its representation as two bent bonds are both mathematically useful but convey no greater meaning to the introductory student than a simple statement that it is a four-electron bond that extends further from the atoms than would a single electron pair. Such an extended electron cloud would then be vulnerable to electrophilic reagents. The simplified molecular orbital theory that predicts that Liz exists, Bez does not, and Oz is paramagnetic fails
to predict that Be0 exists or that HeH does not. The predictive value is limited to pairs of atoms of nearly equal atomic numbers. Where a more elaborate model is explained, the explanation in introductory texts is not carried to the point where it could actually be used. Moreover, there is no value in learning to predict the existence of diatomic molecules when every combination of any interest has been investigated empirically This is especially true when the rules are restricted to homogeneous pairs. There is a real need for a simple explanation of the chemical bond that can be used for low-level decisions but will not have to be unlearned in later courses. Buffer calculations have the appearance of being useful, but at the level we teach they give incorrect answers. Calculating the pH of the standard buffers in the Handbook of Chemistry and Physics by the Henderson-Hasselbach equation gives errors up to 0.5 pH unit. Yet the results of these calculations appear in our texts with two decimal places! There are even occasional suggestions that the calculations be used to design standard buffers to standardize pH meters! The conclusion must be that most chemical educators believe in these calculations--a dreadful indictment of our profession. Yet if students eventually used the calculations for anything, they would find that they had been deceived and would have provided feedback to get our introductory courses corrected. The fact that this has not happened strongly suggests that these calculations are not only incorrect, but also purposeless. I have just once in my professional life had to estimate a n equilibrium constant and the resulting concentrations on a system for which no data was available, starting by adding bond energies to get AG. I found that adding bond energies is a n obsolete approach despite its regular appearance in introductory texts, and used group contributions(3) to get results that were less unreliable. This raises two questions. First, is this block of physicochemical calculations sficiently valuable to be included in an introductory course? If I have used it but once, will a one-year student or even a chemistry major have sufficient use for it to be worth learning, considering that the memory will be vague by the time it is needed? Perhaps. Helshe may remember enough to recognize a problem that could be solved by consulting a chemist. But then, should we teach prospective chemists to use an obsolete approach? The way it now appears in our introductory texts it is a recipe for getting the wrong answer. Adding group contributions is no more difficult than bond energies, so there seems to he no reason not to do it right. Even so, if students are instructed in the use even of group contributions, honesty should compel us to tell them that although the predicted values of AG are usually correct to less than 0.5 kcaVmol they are sometimes in error by a s much as 3 kcdmol. These few items illustrate the questionable usefulness of what we teach and show the need for further critical investigation of the traditional curriculum. No amount of discussion to achieve a consensus would have unearthed all of these problems. Some further subjects that merit investigation would be the limitations of the Nernst equation, the variability of E, the limitations of colligative equations (Raoult's Law gives erroneous vapor pressures for solutes in polymers (4, 511, the constancy of equilibrium constants (other than the ionic equilibria discussed above) and the validity of conversions from rads to rems. I confess to being confused whether Sanderson was correct in asserting ( 6 ) that there is covalent bonding in NaCl crystals, and am not aware that his arguments have been carefully addressed.
We Don't Know What Should be Taught
A common cliche is that we must teach "fundamentals" so that students can use them in the wide ranee of unforeseen problems that they will meet in later l i f e ~ ~ o w e v ear , choice must be made among the fundamentals. Many fundamentals are absent from introductory texts, while others are included that cannot be a ~ d i e to d anv real ~roblem without more sophistication than we are wiliing toteach or students to learn. My inability to understand the brief discussions of corrosion in chemistry texts led me to investigate further and discover that there were useful fundamentals that were not taught. Why is there a potential between two metals that are touching? When would the potential be significant? Why is tin negative in contact with iron while zinc is positive? When soldering copper, how does the choice of solder affect corrosion ofthe assembly? These questions require a t least an explanation of work functions if not of &rLY bands (of w h i h the explanation under mniconductors is also incornprehensihle in all the introductory texts 1 have consultedr. The relation between this Volta ootential between touching metals and the reduction poten&alof the metals is a useful synthesis of two concepts. Why, when aluminum and iron both form stable coatings of oxide in contact with air (A1203 or Fe304)does iron rust while aluminum does not? This requires a discussion of the nature and properties of coatings including the reason (7) that &03clings so much more firmly to aluminum than Fe304does to iron. Nothing was included in my education about coatings, probably because very little was known, but is that an excuse for keeping a new generation in ignorance? I t is reeularlv ex~lainedwhy rust amears at a point on iron t h i t is remot;! from the point wheie oxidation occurs. but it is seldom exdained whv the oxidation occurs there. ~ h requires s a dis&ssion ofthe lfferent work functions at different cwstal faces (in this case of the iron wanules) or a t cracks and edges. Pourbaix diagrams would be useful if explained more carefully than they are in most reference works. Examination questions could then involve using Pourbaix diagrams to predict corrosion or passivity We do not explain how iron can rust below paint where there should be no water or oxygen to facilitate corrosion even if the anode is elsewhere. This auestion needs a treatment of diffision through polymers,'a subject that also is relevant in k e e ~ i n efish alive in ~ l a s t i baes. c instability of solutions in plastic bottles, and semip&neable membranes. We have chosen to teach fundamentals that are of little value to the student while neglecting fundamentals of greater value. This approach to the choice of fundamentals should be pursued by examining other areas of general interest and finding what chemical fundamentals are necessary to their understanding. Such subjects might be the formation of rocks. the oreservation of food. stellar svnthesis. atmosphe& chemistry, biomoleculeiin interstkllar space, nature and origin of the earth's mantle, forensics, archeological chemistry, degradation of environmental pollutants, the ereenhouse effect. and the new materials now being d e s h e d by materials scientists. Don? misunderstand: i am not sueeesting that these subiects should be included in introdu%ry ch&nistry. I am su:fgesting that a thorough exammation of these subircts will tell us what fundamentals a student needs in preparation for further study, and which fundamentals are not useful at this level. ~
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Research is Needed to Define What Should be Taught
Students are required to study chemistry by their major departments on the supposition that it will provide a necVolume 69 Number 3 March 1992
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essary foundation for study of their chosen area. We should find out what chemistry will provide that foundation. One way is to ask faculty in those major departments, but it has been my experience that they have seldom given the matter any thought. Kreyenbuhl and Atwood (8)have approached this with a questionnaire addressed to chemists and engineers: study of their responses suggested little change in cumculum, but my researches suggest a modification of that conclusion. I have re~orted(2) a study of texts of other disciplines requiring one year of chemistry This suggested drastic reduction in the detail of what was covered in many areas such as acid-base chemistry and stoichiometry, even though the subjects themselves were important a s Kreyenbuhl and Atwood (8) have shown. The broad and surprising conclusion from this study was that there is little value in the heavily quantitative treatment that we provide in introductory chemistry. This accounts for the anomaly considered above that no protest has been made about our teaching buffer calculations that give the wrong answers. althoueh buffers are of the first importance. The calculations arean unimportant aspect of biffers and represent little more than an intellectual hoop for students to jump through. Students recognize this, and respond with a Table 1. Prerequisites for Biochemistry
Unoerstand organlc strun~ralform~lae Remgn ze a im~tedrange of Ibnnional groJps.specifically
-NH2
-C02H m4
-OH
ester
\ /c=o
Conoensat on poiymer~zat~on and dlmerlzar on Some elementary b~ocherrcDNA,proteins, sJgars, enzymes Energy \H, A S , AG Bond energy. Energy ot vaporlzallon lnteriolecuiar attraction: dipole-dipoie and H-bond Energy-distance curves Isotopes, radioactivity, background radioactivity Emission and absorption of radiant energy Catalvsis ,-rep^ sqonand attracuon beween ions
Bronsled acwoase concept Equilibrium and Keq Bond rotation Coordinate covalence Oxidation as e-transfer Shapes of molecules (VSEPR)
Table 2. Not Prerequisite to Biochemistry
Nature of the chemical bond: (resonance,orbitals, quantum numbers) Calculations: (stoichiometry,pH, AG, colligative, Hess's Law, calorimetry) Balancing equations Specaic heat Paramagnetism Ionization energy and electron affinity Gas ~ Laws ~ Crystals and glasses Bragg equation Work and AG Electrolysis. Faraday, galvanic cells, Nernst Nuclear fusion.Lssion, dating, magic numbers, neutron activation. band of stabiliw Fahrenheit ~
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Journal of Chemical Education
dislike of what we teach and contempt for what we foist on them as education. Since that study I have worked my way through a biochemistry text written for biochemistry majors. In view of the organic chemistry prerequisite and physical chemistry co-requisite for biochemistry, the actual demand on the student's preparation was surprisingly modest. The necessary prerequisites are listed in Table 1.More interesting is the listing in Table 2 of subjects that are unnecessary for the study of biochemistry. If this kind of evaluation is continued into other areas, for example organic chemistry, pharmaceutical chemistry, and analytical chemistry, we may eventually find that some of these subject areas that are not needed for biochemistry also have little or no use elsewhere. The general finding that a nonquantitative approach is sufficient is true here as elsewhere, and surprisingly little understanding is needed of the nature of the chemical bond. NonprofessionalObjectives A colleee education is resumed to ~rovidea foundation for a stuzent's personal,'intellectual,~ndcivic life as well as professional life. What chemistry is useful in these nonprofessional areas? How do we find out? There are a number of ~ublicationsaimed a t scientifically literate nonscientists. They can provide a guide to what an educated readership finds useful, because they would be out of business if their judgment on this were faulty. Accordingly, I have examined a number of issues of Science News and of ScientificAmerican to see what areas are of interest and what are their prerequisites. The results are in Tables 3, 4, and 5, which are remarkable for their brevity. It would be useful to consult newspapers (and the last page of Chemical and Engineering News) to discover the subjects where a knowledge of chemistry would have prevented a journalistic or political error. This might have prevented the embarrassment of the politician who insisted that it was insufficient that the pH of a lake was 6 Table 3. Subjects of Interest, as Judged from Science News
Corrosion Materials Biochemistry S ~ a c Chemistw e
Archwlogy and Chemistry Environmental Chemistry Nature of Maner
Table 4. Prerequisites for Reading Science News
Recognize simple formulae such as CO,, NH, Qualitative eiectrochem Biochemistry Absorption and emission of light Free radicals Mechanism of solution Units of radioactivity (Ci, rem) Table 5. Prerequisites for Reading Scientific American
Blochem (a nle more than one chapter) pH, meanmg of a c o base, sa I, aclo sa 1 Mecnan sm of so ut on of salts n aclo or compexanrs Fractional crystallization Atomic number, isotopes, shielding Stereoisomerism Crvstal Structure
a n d t h a t no expenditure should be spared until i t was reduced to zero. O r t h e airline stewardess who insisted that there was no need for passengers to extinguish cigarettes while she administered oxygen, because oxygen doesn't burn.
of chemistry that is then never used in the remainder of the text or is used only at a qualitative level. The keywords must then be cross-checked to see if the subjects are really used, and the text must be examined bv a chemist to confirm this bv. a . orofessional judgment. This studv should extend hevond the limited ranee of auhiect arras in reference 2, which, Inter aha, omitted ~nginerring.It might wcll lrnd us to discover that surface chrmlstry, the methodology of hreakmg the genene code or of using it. the suencqh and permeability of plastics, the modem technology of filtration, or the principles of hydrophobicity are more important than some now popular subjects.
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Imaginative Stuff The intricate programs now available for molecular modelling make i t possible for students to examine computer models of wmplex molecules. This kind of experience must be treated cautiouslv. When a becrinning chemist sees a model of a protein,
